Organic EL (electroluminescent) displays are conventionally known as being thin display devices featuring high image quality and low power consumption. The organic EL display has a plurality of pixel circuits arranged in a matrix, each circuit including an organic EL element, which is a self-luminous element driven by current, and a drive transistor for driving the element.
The method for controlling the amount of current to be applied to a current-driven display element such as the organic EL element is generally classified into a constant-current control mode (or current-programmed drive mode) in which the current to be applied to a display element is controlled by a data signal current applied to a data signal line electrode of the display element, and a constant-voltage control mode (or voltage-programmed drive mode) in which the current to be applied to a display element is controlled by a voltage corresponding to a data signal voltage. Among these modes, when the constant-voltage control mode is used for display on an organic EL display, it is necessary to compensate for current reduction (luminance decrease), which is caused by variations in the threshold voltage among drive transistors and high resistance due to deterioration of organic EL elements over time. On the other hand, in the constant-current control mode, the current value of the data signal is controlled such that current is applied to the organic EL element at a constant level regardless of the threshold voltage and the internal resistance of the organic EL element, and therefore compensation as mentioned above is normally not required. However, more drive transistors and lines are used in the constant-current control mode than in the constant-voltage control mode, resulting in a reduced aperture ratio, hence wide use of the constant-voltage control mode.
Here, there are various conventionally known types of pixel circuit that perform compensation operations as mentioned above in the configuration employing the constant-voltage control mode. Japanese Laid-Open Patent Publication No. 2006-215275 describes a pixel circuit 80 shown in FIG. 21. The pixel circuit 80 includes TFTs (thin-film transistors) 81 to 85, a capacitor 86, and an organic EL element 87. To perform writing to the pixel circuit 80, first, the TFTs 82 and 84 are brought into ON state, thereby initializing a gate-source voltage of the TFT 85 (drive transistor). Next, the TFT 84 is brought into OFF state, and then the TFT 83 is brought into OFF state, so that a threshold voltage of the TFT 85 is held by the capacitor 86. Subsequently, a data potential is applied to a data line DTL, and also the TFT 81 is brought into ON state. By controlling the TFTs in this manner, it is rendered possible to compensate for variations in the threshold voltage among TFTs 85 and high resistance (and resulting current reduction), which occurs due to deterioration of organic EL elements 87 over time.
The pixel circuit 80 is connected to the data line DTL, four control lines WSL, AZL1, AZL2, and DSL, and three power lines (lines for Vofs, Vcc, and Vss). In general, as the number of lines (particularly, control lines) connected to the pixel circuit increases, the circuit becomes more complicated, resulting in increased production cost. Therefore, the pixel circuit described in Japanese Laid-Open Patent Publication No. 2006-215275 has the TFT 82 or 84 connected at its source terminal to the control line WSL. Also, Japanese Laid-Open Patent Publication No. 2007-316453 describes a pixel circuit in which the TFT 82 is connected at its gate terminal to a control line WSL for an immediately preceding row. By commonly using a line as both the control line and the power line, the number of lines used can be reduced.
Japanese Laid-Open Patent Publication No. 2007-310311 describes a pixel circuit 90 shown in FIG. 22. The pixel circuit 90 includes a TFT 91, a TFT 92, a capacitor 93, and an organic EL element 94. To perform writing to the pixel circuit 90, first, the TFT 91 is controlled to be in ON state. Next, an initialization potential is applied to a power line DSL and thereby to an anode terminal of the organic EL element 94. Then, a power potential is applied to the power line DSL, so that a threshold voltage of the TFT 92 (drive transistor) is held by the capacitor 93. Subsequently, a data potential is applied to a data line DTL. Such provision of the initialization potential through the power line makes it possible for a small number of elements to achieve compensation for variations in the threshold voltage among TFTs 92. Japanese Laid-Open Patent Publication No. 2007-148129 describes a pixel circuit to which an initialization potential is provided through a power line and a reference potential is provided through a data line. Japanese Laid-Open Patent Publication No. 2008-33193 describes a pixel circuit in which a compensation operation is performed in a plurality of horizontal periods before writing.